1. Field of the Invention
The present invention relates to active optical devices and, in particular, to active optical devices which sense phase information and then alter the phase characteristics of a beam of optical light on the basis of that sensed phase information.
2. Description of Related Art
For certain applications, it is desirable to have lenses, mirrors or other optical elements which have optical properties that are alterable on demand. For example, one application may require a focusing mirror whose radius of curvature can be switched between two different values. A more specialized application might require a different mirror which has a deformable surface that alters the phase distribution of light reflected from the mirror. Such "alterable" optical elements are generally referred to as adaptive optics. For many applications, high speed and low power performance are necessary to the successful implementation of adaptive optics.
One application of adaptive optics is to improve the imaging performance of optical systems by compensating for optical imperfections or aberrators present in the system. For example, imaging performance may be improved by using adaptive optics to alter the phase distribution of the observed beam of light. If a beam of light carrying an image passes through an aberrator, the aberrator will alter the phase distribution of the beam and distort the image carried by the beam. When the aberrated light beam is reflected from a suitably deformed adaptive optic element, the phase distribution of the beam of light is altered in accordance with the deformation of the element's surface. Under appropriate conditions, reflection from the deformed element can subtract substantially all of the phase variation from the aberrated beam of light. Subtracting the phase variation from the light beam has the effect of correcting the image carried by that beam of light, thereby improving the imaging performance of the system.
Using adaptive optics to correct distorted images or to remove aberrations from beams of light has several useful applications. For example, the resolving power of optical telescopes operating within the earth's atmosphere is reduced by atmospheric distortion of the light incident from distant objects. Such atmospheric distortion can be caused by atmospheric turbulence or other local variations in the air's refractive index. Generally, atmospheric distortions such as these cannot be compensated for by fixed mirrors or lenses because the magnitude and pattern of the distortion varies in time. Thus, an adaptive optics solution is highly desirable. Such an adaptive optics solution typically requires some means of measuring the phase characteristics of the atmospheric aberrator. The measured phase characteristics would then be imposed on an alterable optical element to compensate for the effects of the aberrator.
One technique for measuring the phase information characteristic of a given aberrator is to create a hologram of that aberrator. Such a holographic technique is described by Munch and Wuerker in Applied Optics, Volume 28, pages 1312-17, Apr. 1, 1989, where it is applied to measuring the constant aberrations introduced to a beam of light by a low quality telescope objective. In that work, a signal beam comprised of plane wave light was passed through the imperfect objective of a telescope. The imperfect objective modulated the signal beam, introducing phase variations to the formerly constant phase wave front. Next, the aberrated signal beam was combined with a plane wave reference beam that was spatially and temporally coherent with the signal beam. The two beams interfered, creating interference fringes that were recorded within a photographic plate. The resulting hologram was used to correct the aberrations of the telescope by placing the hologram in the optical path of the telescope.
When a hologram is formed within a photographic plate, the interference fringe pattern incident on the plate is recorded within the emulsion on that plate. After the plate is developed, the intensity distribution is converted into a quasi two-dimensional pattern of varying indices of refraction. This pattern can be considered one type of diffraction grating which can be used in a transmission mode or, under appropriate conditions, in a reflection mode. When a beam of light is passed through this recording of the interference pattern, the beam of light will be modulated with the original phase distribution. Thus, the original phase distribution can be subtracted from the aberrated beam, correcting the image carried by that beam.
This holographic image correction technique has the disadvantage of being implemented with photographic plates. Photographic plates usually require chemical development which can introduce a several minute delay before a usable hologram is produced. A further disadvantage associated with developing photographic plates is the fact that the plate usually has to be moved from the position where it was exposed to be developed. This can introduce the need for careful alignment of the developed hologram before it can be used.
A second example of the holographic phase recording technique is discussed by Karaguleff and Clark in Optics Letters, Volume 15, pages 820-22, Jul. 15, 1990. That work used a nematic-phase liquid crystal device both to record a hologram of an aberrator and as an optical element to compensate for aberrations introduced into an object beam by that same aberrator. In this manner, the nematic-phase liquid crystal device acted both as holographic media and as an adaptive optic element. Two physically distinct operations were performed, almost simultaneously, by the liquid crystal device. First, the hologram of the aberrator was created and recorded in the liquid crystal device. Next, the hologram was used to correct aberrations introduced to an independent beam of light by the same aberration.
A beam splitter was used to divide a plane wave beam of light into a signal beam and a reference beam. The signal beam was passed through an aberrator which distorted the beam, modulating the plane wave light with the phase variations characteristic of that aberrator. Then both the aberrated signal beam and the plane wave reference beam were directed to one surface of the liquid crystal device. The two beams interfered across the surface of the liquid crystal device, creating a two-dimensional interference fringe pattern which was recorded by the liquid crystal device. High intensity portions of the fringe pattern produced local increases in the temperature of the liquid crystal. Because a nematic-phase liquid crystal's index of refraction is temperature dependent, the fringe pattern formed by the combination of the aberrated signal beam with the reference beam produced a hologram comprised of localized index of refraction variations in the liquid crystal device.
The refractive index hologram was utilized in the following fashion to perform image correction on an aberrated light beam. A different, independent beam of light was passed through an object and then through the aberrator that was used to create the refractive index hologram. This "object" beam thus carried the image of the object, distorted by the aberrator. Next, the object beam was passed through the refractive index hologram in an appropriate manner so that the phase variations recorded in the refractive index hologram were subtracted from the object beam. This subtraction acted to compensate for the aberrations in the object beam, causing the aberrations to be removed from the image of the object carried by the object beam. Thus, the refractive index hologram was used to perform image correction.
There are substantial disadvantages associated with this method of image correction. Since the interference fringe pattern is recorded thermally, relatively high optical power densities are usually needed to create this sort of hologram. This high incident power requirement can be reduced by heating the entire liquid crystal device to a temperature close to the liquid crystal transition temperature. In this region, the index of refraction of the liquid crystal device is generally the most sensitive to changes in temperature. While this technique offers improved performance, the higher temperature can sometimes lead to thermal runaway conditions in the liquid crystal device which can substantially degrade the quality of holograms produced in those devices. Furthermore, light scattering is substantially greater when this device is operated near the liquid crystal transition temperature.
An additional disadvantage with the above technique is that the amplitude of the refractive index variations that comprise the refractive index hologram decreases in time. The time constant for this amplitude decay is typically on the order of thirty microseconds. Thus, any image correction usually must be performed rapidly. The thermal runaway problem typically places a limit on the repetition rate that is compatible with this device, as well. Thus, this nematic-phase liquid crystal device is often poorly suited to applications requiring steady-state or near steady-state operation.
Typically, adaptive optical elements such as deformable or segmented mirrors, or those described above, have an essentially analog character in that the deformable or adjustable elements vary continuously and smoothly. For example, in the nematic-phase liquid crystal device, the refractive index variations have a substantially sinusoidal character on a local scale and the amplitude of the variations can vary widely. The refractive index does not change as a step function, rather it changes relatively slowly and continuously. In certain applications it may be desirable to have sharper phase modulating characteristics than are typically available in previous types of adaptive optics elements.